1. Field of the Invention
This invention relates to a silicon nitride (Si3N4) substrate with high mechanical strength, high insulation resistance, and high thermal conductivity, and a manufacturing method for the same. This invention also relates to a silicon nitride wiring board and semiconductor module using the above-mentioned silicon nitride substrate.
2. Description of the Related Art
In recent years, power semiconductor modules (IGBT, MOSFET, etc.) in which high current or high voltage operation is possible, are used as an inverter for electric motors. As a substrate used for the power semiconductor module, insulating ceramics substrates made of aluminum nitride or silicon nitride are used. The conductive metal plate in this substrate which serves as a circuit on one face (upper surface) is formed, and the metal plate for heat dissipation is formed on another face (undersurface).
The ceramic wiring boards with this structure are used widely. The copper plate or the aluminum plate is used as these metal plates. Semiconductor devices etc. are mounted on the upper face of the conductive metal plate used as a circuit and the active brazing filler method is used for the connection between the ceramics substrate and the metal plates. What is called a direct bonding copper method, in which the copper plates are bonded directly, is also used.
However, in many cases, in the power semiconductor module using the ceramics substrate on which the metal circuit plate and the metal heat sink were bonded, thickness of the metal circuit plate and of the metal heat sink is made 0.3-0.5 mm, so that high current can be sent through them. When copper with high thermal conductivity is especially used for the metal circuit plate and the metal heat sink, thermal expansion coefficient of copper differs from ceramics greatly. Therefore, if copper is bonded to ceramics, heat stress will be generated in the cooling process after the connection of these metals and ceramics. This stress is generated as compression or tensile residual stress near the junction of the ceramic substrate. This residual stress makes a ceramic substrate cracked, or makes isolation voltage low. Or exfoliation of the metal circuit plate and the metal heat sink may be generated.
Although an aluminum nitride substrate has high thermal conductivity as a ceramic substrate, its mechanical strength is low, and the reliability in mechanical strength is low and the substrate is hard to be used. On the contrary, a silicon nitride substrate has high thermal conductivity, and has also excellent mechanical strength, high fracture toughness, and high heat-resistant fatigue characteristics. Therefore, various structures of silicon nitride substrates are proposed as shown below.
For example, in the conventional example, there is a silicon nitride substrate with 86-99 mol % of silicon nitride content, also containing other elements. Here, as the doped element, one or more elements selected among the group of yttrium (Y) and rare earth (RE) element of lanthanide are doped with 1-10 mol % of oxides of these elements. Moreover, one or more elements selected from lithium (Li), magnesium (Mg), calcium (Ca), titanium (Ti), zirconium (Zr), or hafnium (Hf) are doped with 0-4 mol % of oxide of these elements. Moreover, aluminum (Al) is also doped with 1000 ppm or less. The percentage of β type silicon nitride in this silicon nitride is 30% or more. This silicon nitride composite is manufactured by sintering at temperature of 1700-2000° C. in pressurized nitrogen atmosphere at pressure of 1 MPa or less (for example, refer to Japanese Patent No. 2002-29849). Hereafter, this technology is called the 1st conventional example.
Moreover, there are the following as other manufacturing method of a silicon nitride substrates. β type silicon nitride used as sintering aids and as seed crystals is added to the silicon nitride powder. The obtained mixed powder is distributed to carrier fluid, and slurry is prepared. After making forming body of β type silicon nitride by this slurry, the forming body is densified, after degreasing. Furthermore, after sintering at 1700-2000° C. at pressure of 1-100 atm nitrogen atmosphere, silicon nitride substrate is obtained. This silicon nitride substrate has the structure with some orientation and has thermal conductivity of 100-150 W/m·K along the orientation (for example, refer to Japanese Patent No. H09-165265). Hereafter, this technology is called the 2nd conventional example.
Moreover, there are the following as other silicon nitride substrates. In this silicon nitride substrate, the sum of the content of oxygen (O), aluminum (Al), calcium (Ca), and iron (Fe) is 1000 ppm or less. And the silicon nitride particles whose diameters along a short axis are 2 micrometers or more are contained. Some of such particles with diameter of 2 micrometers or more along the short axis, have orientation along the substrate plane direction or along the thickness direction (for example, refer to Japanese Patent No. 2001-19555). Hereafter, this technology is called the 3rd conventional example.
In the silicon nitride substrate composed of the silicon nitride substrate by the 1st above-mentioned conventional example, it is not taken into consideration about the orientation of β type silicon nitride at all. Therefore, there is a limit in raising thermal conductivity and fracture toughness. On the other hand, thermal conductivity and fracture toughness can be raised by carrying out orientation of the β type silicon nitride like the 2nd and 3rd above-mentioned conventional examples, because high thermal conductivity is acquired along the direction of the orientation since the number of grain boundaries spoiling the heat conduction is decreased. On the other hand, crack is hard to be grown along the perpendicular direction to the orientation, because the crack may have orientation by the orientation of β silicon nitride. Therefore, fracture toughness is improved.
However, in the above-mentioned 2nd and 3rd conventional examples, while the thermal conductivity along the direction in the plane will become high, the thermal conductivity along the thickness direction becomes low, if the degree of orientation fa in the direction of the plane of the silicon nitride substrate 1 is too high as shown in FIG. 7. Here, below, the degree of orientation fa in the direction of the plane is called “degree of in-plane orientation.” In the example of FIG. 7, fa is around 1. Therefore, in the semiconductor module composed of the silicon nitride wiring boards comprising this silicon nitride substrate, metal circuit plate, and metal heat sink, and semiconductor devices, thermal resistance cannot be made small. In addition, the meaning of the degree of in-plane orientation fa is mentioned later. On the other hand, in the 3rd above-mentioned conventional example, if the degree of orientation along the thickness direction of the silicon nitride substrate 2 is too high as shown in FIG. 8, the thermal conductivity along the thickness direction will become high. However, in-plane thermal conductivity becomes low. Here, in the example of FIG. 8, the degree of in-plane orientation fa is around −1. Moreover, when manufacturing the above-mentioned silicon nitride wiring board, the bonding temperature of the metal circuit plate made of copper (Cu) is about 800° C. In the cooling process from this temperature, or after the heat cycle while the semiconductor module is working, thermal stress is generated due to difference of the thermal expansion coefficient between copper and silicon nitride. Therefore, as shown in FIG. 9, the crack which goes into the silicon nitride substrate 31 from the perimeter part of the metal circuit plate 32 formed on the silicon nitride substrate 31 may be generated easily. Therefore, exfoliation of the metal circuit plate 32 may arise easily, and the reliability of the silicon nitride wiring board 34 is low. After all, in the conventional silicon nitride substrate, the problem concerning the degree of in-plane orientation fa, thermal conductivity, and fracture toughness was not examined. Moreover, what indicated a means to solve these problems is not found. In FIG. 9, the metal circuit plate 32 is bonded to the upper surface of the silicon nitride substrate 31, the metal heat sink 33 is bonded to the undersurface, respectively, and the silicon nitride wiring board 34 is constituted. Moreover, the semiconductor device 35 is mounted on the upper face of the metal circuit plate 32, and the semiconductor module 3 is constituted.
The present invention is made in view of the situation mentioned above, and an object of the invention is to provide a silicon nitride wiring board and a semiconductor module using the silicon nitride substrate which can solve the above subjects and a manufacturing method thereof.